1. Field of the Invention
The present invention relates to an image reader for measuring light reflectivity or transparency of an image bearing member and more particularly to an improvement in performance and measuring accuracy of the image reader.
2. Description of the Prior Art
There are two types of image readers used conventionally in the field of printing: image readers which are intended for reading an image as detailed as possible to convert the image into processible data for a computer and the like, and image readers which are intended for measuring the area of either the white or black part of a binary monochrome image. The former is generally known as an image scanner. The latter is known as an ink pattern area meter, which is used for measuring the proportion (an ink pattern area rate) of a picture part (the part supplied with ink) in each portion of a printing plate or an original film to the whole printing plate or original film in order to establish the amount of ink supplied to a printing machine before a printing process.
The ink pattern area meter is intended for measuring an average ink pattern area of each of tens of small regions into which a printing picture plane is divided. Because of this intention, spatial resolution of the ink pattern area meter is low in comparison with the image scanner. The value of the spatial resolution is about several mm to 100 mm according to the printing machine to be employed. However, the accuracy of a measured value, that is, the absolute value of the difference between a real ink pattern area rate and the corresponding measured ink pattern area rate is severely required to be 1 to 5% in general. Particularly in printing of printed matter with a small ink pattern area rate (e.g., business forms), it is desirable to minimize errors in measurement.
An example of the image readers which are intended for measuring the ink pattern area rate is an apparatus disclosed in Japanese Patent Application Laid-Open No. 57-64102. A measuring head of this apparatus is described hereinafter referring to FIGS. 15 and 16. FIG. 15 is a cross-sectional view of the measuring head of the apparatus taken along the plane perpendicular to a main scanning direction X. FIG. 16 is a cross-sectional view taken along the line A--A of FIG. 15. In the description, a reference numeral or character which designates each component in the figures is provided with no subscript when the component is indicated as a representative of the whole (for example, photodiodes 6). A subscript is attached to the reference numeral or character when the individual components are specified. (For example, a plurality of members 6.sub.-1, 6.sub.-2, 6.sub.-3 . . . 6.sub.-n are referred to simply as 6.sub.-1 to 6.sub.-n.) The same is true for other figures.
As shown in FIGS. 15 and 16, the measuring head comprising a light source unit 2 and a light detecting unit 3 is provided above a printing plate 1 to be measured. The light source unit 2 comprises a pair of fluorescent lamps 4 each extending in the main scanning direction X and a reflecting plate 5 which covers the fluorescent lamps 4 from above down to the both sides thereof. The light detecting unit 3 comprises a plurality of photodiodes 6.sub.-1 to 6.sub.-n arranged linearly in the main scanning direction X on a plane parallel to the printing plate 1, obstructors 7 disposed between the adjacent photodiodes 6.sub.-1 to 6.sub.-n for partition, and a stopping plate 8 attached to the lower ends of the obstructors 7 in parallel with the printing plate 1. The stopping plate 8 is formed with apertures 9.sub.-1 to 9.sub.-n at positions corresponding to the respective photodiodes 6.sub.-1 to 6.sub.-n. In FIG. 15, numeral 10 designates a predetermined section measured by a single photodiode 6, and 11 designates light which is projected from the fluorescent lamps 4, is reflected by the predetermined section 10, passes through the aperture 9 of the stopping plate 8, and is incident on the photodiode 6.
In general, light reflectivity of an offset printing plate in a picture part is different from that in a non-picture part. The apparatus makes use of this characteristic. That is, by illuminating predetermined sections 10 in the printing plate 1 with the light of the fluorescent lamps 4, the ink pattern area is measured based on the amount of light reflected by the predetermined sections 10.
In the apparatus, the light 11 projected from the fluorescent lamps 4 is reflected by the respective predetermined sections 10.sub.-1 to 10.sub.-n in the printing plate 1, where the light is decayed according to the reflectivity thereof, and thereafter the light passes through the apertures 9.sub.-1 to 9.sub.-n of the stopping plate 8 and is incident on the photodiodes 6.sub.-1 to 6.sub.-n. Since the photodiodes 6.sub.-1 to 6.sub.-n are arranged linearly in the main scanning direction X, a strip-like region extending in the main scanning direction X is measured at one time. Thus, each strip-like region extending in the main scanning direction X is measured in series, while the measuring head consisting of the light source unit 2 and the light detecting unit 3 travels in a sub-scanning direction Y which is perpendicular to the main scanning direction X, thereby the ink pattern area of the whole printing plate 1 can be measured.
In the conventional apparatus as above mentioned, however, the predetermined sections 10 to be measured are determined by the shapes and positional relations of the apertures 9 of the stopping plate 8 and the effective photosensitive surfaces of the photodiodes 6. A problem is that errors in the shapes and positions thereof affect measuring accuracy directly.
The stopping plate 8 with an error of several .mu.m to tens of .mu.m can be manufactured by means of high-accuracy mechanical working, etching, laser cutting and the like. The effective photosensitive surfaces of the photodiodes 6 can be shaped with high accuracy by the technique of IC manufacture. However, the dimensional error in the shape of a package of photodiodes 6 is quite large, that is, about hundreds of .mu.m in common. It is quite difficult to align tens of such photodiodes 6 with accuracy. Either the rotation or the inclination of a square element surface of each photodiode 6 affects the shape of the predetermined sections 10 to be measured. In the apparatus having such a structure, it has been difficult to achieve the high-accuracy measurement.
In the conventional apparatus as above mentioned, the predetermined sections 10.sub.-1 to 10.sub.-n measured by the respective photodiodes 6.sub.-1 to 6.sub.-n are divided so that the overlap of the predetermined sections 10 measured by the adjacent photodiodes 6 is as small as possible. When the predetermined sections 10.sub.-1 to 10.sub.-n measured by the respective photodiodes 6.sub.-1 to 6.sub.-n are out of position or out of shape with the production of a manufacturing error of optical parts, a dead area in the predetermined sections to be measured and an extremely high sensitivity region are liable to be generated. Particularly the dead area in the predetermined sections to be measured results in a failure in reading a line. In printing, ink must be supplied to a region having a picture part which includes only a single line. The failure in reading a line is more problematic than the error in the absolute value of a measured value and is a considerably serious problem particularly in printing of printed matter with a small ink pattern area such as business forms and the like.
When the apparatus is designed to reduce the overlapping part of sensitivities as the conventional apparatus, the effective photosensitive area of the photodiodes 6 is decreased. Accordingly, the amount of light which is incident on the photodiodes 6 is decreased, and a disadvantage is that the apparatus is subject to the influence of electric noises.
When the overlapping part of the sensitivities of the adjacent photodiodes 6 is small, the influence of a manufacturing error and assembling error of the parts on the measuring error is large in not a few cases.
In such an apparatus, it has been difficult to sufficiently achieve the intention of the ink pattern area meter.